People who purchase health care individually could face premium hikes of up to 13 percent under the Senate health care overhaul, but the vast majority of Americans would not be hit by rate increases, according to a new report that drove the first day of contentious debate on the Democrats' legislation.

The Congressional Budget Office (CBO), a nonpartisan budget-keeper, found that the approximately 18 million people who buy insurance on the individual market and qualify for new income-based tax subsidies would see their costs fall by more than half; the 14 million people who don't qualify would see their costs jump 10 to 13 percent.

The CBO also found that people who get their insurance coverage through an employer or large group plan - about 70 percent of Americans - would see premium prices remain the same or fall by up to 3 percent. It based its estimates on what premiums would look like in 2016, and warned they are rough estimates.

Lawmakers on both sides used the report to bolster their arguments as the Senate officially began debate on the bill, a 10-year, $979 billion plan to establish a government-run insurance program, set up tax credits to help the poor meet a mandate that nearly all Americans purchase insurance, and reform the insurance industry.

"The analysis we received [Monday] indicates that whether you work for a small business, a large company or you work for yourself, the vast majority of Americans will see lower premiums than they would if we don't pass health reform," said Sen. Max Baucus, Montana Democrat and chairman of the Finance Committee.

For Republicans, the report highlighted how the measure will not address the rising, high cost of care President Obama vowed to tackle.

"After 2,074 pages and trillions more in government spending, massive new taxes and a half-trillion dollars in cuts to Medicare for seniors, most people, according to the Congressional Budget Office, will end up paying more or seeing no significant savings," said Minority Leader Mitch McConnell of Kentucky. "This is not what the American people are asking for."

As the debate began Monday, Republicans objected to early attempts by Democrats to establish rules on the debate and amendments, signaling just how contentious the discussion is likely to be.

Democrats proposed a rule, called a unanimous consent agreement, preventing lawmakers from using money from the Social Security fund or a new in-home care insurance program for other reform programs, and another that would require lawmakers to post amendments online before formally offering them. Republicans said the first rule was flawed because it excluded Medicare and the latter would restrict the minority party's ability to file amendments.

"This is not a good way to start this debate," responded Majority Leader Harry Reid, Nevada Democrat.

The debate is likely to run through the end of this year and possibly into next year. Mr. Reid told lawmakers that weekend sessions are going to be necessary to vote on the dozens of amendments expected from both Democrats and Republicans.

"We're going to have to work Saturdays and Sundays," he said in a speech on the Senate floor. "This crisis - and yes, it's a real crisis - is simply too hazardous to our country's and our constituents' health not to work as much and as long as we have to."

On Monday, Democrats offered their first amendment, from Sen. Barbara A. Mikulski of Maryland, which would require insurance companies to cover certain preventive care screenings for women's health.

Republicans' first amendment was a proposal from Sen. John McCain of Arizona to eliminate about $500 billion in cuts to the Medicare program that Democrats say would come from waste and fraud. One of their key talking points on health care reform, Republicans argue there is no way to cut $500 billion in waste without having an impact on seniors' health care.